1. Field of the Invention
The invention relates to isolation amplifiers, and particularly to isolation amplifiers having a very high degree of isolation of the output from the input and having very high signal to noise ratio.
2. Description of the Prior Art
Isolation amplifiers have been developed for various applications wherein greater isolation between amplifier inputs and outputs is required than is available for conventional instrumentation amplifiers and differential DC amplifiers. Isolation amplifiers are widely used in the medical electronics field for applications wherein it is often necessary that there be an absolute minimum of conductive paths from inputs of the amplifiers to their outputs. For example, in many medical electronics applications it is desirable that no more than roughly ten to twenty microamps be coupled from an amplifier input to its output. The main type of coupling path existing between an input and an output of a present state-of-the-art isolation amplifier is a capacitive coupling, which may be of the order of 20 picofarads.
A number of different types of isolation amplifiers are commonly used, including transformer coupled isolation amplifiers, optically coupled isolation amplifiers and pulse width modulation (PWM) isolation amplifiers. All of these types of isolation amplifiers have the shortcoming that they are very expensive and can only be utilized in applications wherein it is imperative that a high degree is isolation be maintained between an isolation amplifier outputs and inputs. However, there are many other areas wherein isolation amplifiers could be widely utilized if they were less expensive than they presently are. One technique presently utilized where low cost is essential to provide a high level of isolation between a data source circuit and a data utilization circuit is referred to as the "flying capacitor" technique. In accordance with the flying capacitor technique, reed relays coupled to form a double pole, double throw switch are first activated to connect a capacitor to the data source circuit, which charges up a capacitor. The reed relays are then switched to disconnect the charged capacitor from the source circuit and to connect the capacitor to a data utilization circuit. A typical cost figure for implementation of the flying capacitor technique is approximately $10.00 per data channel. The technique suffers the shortcomings that reed relay systems are slow, bulky, unreliable, and frequently cause deterioration due to varying resistances of the reed relay contacts. Thus, there is a presently unmet need for a low cost, highly reliable isolation amplifier.
Accordingly, it is an object of this invention to provide a highly reliable, low cost isolation amplifier which overcomes the shortcomings of the flying capacitor isolation technique.
A major factor contributing to the high cost of the above mentioned prior art isolation amplifiers is that they require expensive, electrically floating power supplies capable of delivering a substantial amount of current. Such power supplies alone add at least roughly $20.00 to the cost of present state of the art isolation amplifiers.
Accordingly, it is another object of the invention to provide a low cost, highly reliable isolation amplifier which includes a power supply system which is much less expensive than power supply systems for prior art isolation amplifiers.
It has been found that certain types of electronic analog equipment can sometimes produce erroneous signals when the equipment is utilized in the presence of a high level of electromagnetic interference (EMI). This phenomena appears to frequently occur as a result of low frequency "beats" which occur between high frequency signals produced in the electronic equipment and ambient high frequency EMI signals sensed by the electronic equipment. In certain circuit applications, such as nuclear reactor control systems and safety systems, and in certain medical electronic devices, such as devices connected to probes used to monitor responses of a patient during open heart surgery, it is imperative that no erroneous signals be produced as a result of sensitivity of circuitry to ambient EMI signals.
Accordingly, it is another object of the invention to provide a low cost, highly reliable isolation amplifier which is very insensitive to ambient EMI signals.
In the past, substantial reductions in the cost of electronic circuitry has often been accomplished by integration of circuit systems or subsystems onto as few monolithic integrated circuit chips as possible. The fact that no highly reliable isolation amplifiers have yet been integratable onto a small number of economically sized integrated circuit chips is reflected in the present high price of presently available isolation amplifiers.
Accordingly, it is another object of the invention to provide a low cost, highly reliable isolation amplifier which can be implemented utilizing only one or two integrated circuit chips.